1. Field of the Invention
The invention relates to spiral inductors and, in particular, to spiral inductors utilizing a pad metal layer.
2. Description of the Related Art
Spiral inductors are important, performance-limiting components in monolithic radio-frequency (RF) circuits, such as voltage-controlled oscillators (VCOs), low-noise amplifiers (LNAs), and passive-element filters. Quality factor of the inductors, limited by resistive losses in the spiral coil and substrate losses, is expressed as Q=im(Z)/re(Z), wherein Z is impedance of the inductor.
FIG. 1A shows a layout of a conventional spiral inductor. FIG. 1B is a cross section of the spiral inductor 100 in FIG. 1A. As shown, the spiral inductor is formed on a silicon substrate using multilevel interconnects provided in mainstream silicon processes. At least two metal layers are required to form a basic spiral coil 110 and an underpass 120 to return the inner terminal of the spiral coil to the outside. The spiral coil 110 is typically formed with a top metal layer and the underpass 120 with a metal layer lower than the top metal layer. The underpass 120 is connected to the spiral coil such that the inner terminal of the spiral inductor 100 is routed out.
The top metal layer in semiconductor process is typically much thicker than other metal layers and thus the spiral inductor 100 is typically formed with the top metal layer such that quality factor thereof is maximized. Unfortunately, the underpass 120 is typically formed with a thinner metal layer, leading to increased resistance of the spiral inductor 100. As a result, the spiral inductor 100 suffers from resistive loss and quality factor thereof is degraded.